1. Field
This invention relates generally to electronic devices using automatic gain control, and more specifically to analog circuits having cascaded controllable amplifying (gain) stages that are controlled with an automatic gain control system.
2. Related Art
In many applications of electronic circuits, a signal can benefit from signal conditioning such as by using an automatic gain control (AGC) system. In particular, for example, the presence of noise signals and other interference signals can deteriorate the quality of a desired signal. This is especially problematic in sensitive analog circuits having high amplification (or gain), such as found in analog receiver circuits. An AGC system is typically used to try to reduce the gain of unwanted signals while maintaining or increasing the gain of the desired signal.
A measure of a signal quality is sometimes stated as a Signal to Noise Ratio (SNR). Due to high amplification (gain) of desired signals in a particular electronic circuit design, such as by using a series of cascaded gain stages in a line-up, the high overall amplification also increases unwanted signals, e.g., noise and interference signals, to high levels. This overall gain increase can detrimentally affect the performance of the circuit, such as by causing clipping of desired signal at an output of the circuit. An AGC system can be used to adjust the overall gain of the amplification (or gain) stages to try to reduce the levels of unwanted signals while still maintaining or increasing the level of the desired signal. However, selectivity in an analog receiver circuit call create some problems for the AGC system as discussed below.
For receivers with high analog selectivity, AGC switch points can occur much earlier than necessary. These early switch points detrimentally impact the on-channel SNR regardless of whether noise or interference signals are present or whether noise or interference signals are strong enough to create clipping of desired signal.
Another receiver performance metric used, for example, for high-speed downlink packet access (HSDPA or 3.5G) transceivers is the receiver error vector magnitude (EVM) performance under both on-channel signal-only and interferer test cases. HSDPA transceivers require a receiver EVM of around 5% to achieve desired network throughput at signal levels of −60 dBm and higher. Receiver EVM performance is typically required to be 5% under on-channel signal-only test cases at antenna signal levels of −60 dBm and higher. Receiver EVM performance is typically required to be approximately 10% for adjacent channel interferer test cases.
To achieve this type of receiver EVM performance, an AGC in such receivers should maintain loop bandwidths of 1-kHz or less. Maintenance of such narrow bandwidths causes the receiver to be unable to quickly track out large gain errors introduced in the receiver whenever the AGC system alters gain settings. This inability leads to degraded receiver performance, such as under fading channel conditions. Running the AGC systems continuously in a medium or high bandwidth mode of operation significantly degrades receiver EVM performance to an unacceptably large degree of more than 15%.
Known receivers lack accurate power detectors at the various analog cascaded gain stages. This forces the AGC to sacrifice on-channel SNR and EVM performance by switching out gain earlier than necessary to prevent noise and interfering signals from compressing signal and/or creating IMD (Inter-modulation Distortion) products.